a, FTIR spectra of PBTTT-C14 thin films: black, pristine; orange, F4TCNQ-doped; light blue, anion-exchange-doped with EMIM-TFSI; red, anion-exchange-doped with Li-TFSI; and blue, anion-exchange-doped with Li-BOB. b, Magnified FTIR spectra from 2,050 cm−1 to 2,250 cm−1. C≡N stretching (around 2,190 cm−1) appears only in the case of the F4TCNQ-doped film. c, Magnified FTIR spectra from 1,650 cm−1 to 1,950 cm−1. The doublet peak assigned to the C=O stretching mode (around 1,800 cm−1) in carbonyl groups in BOB− (ref. 24) is generated only in PBTTT films anion-exchange-doped with Li-BOB. This suggests that the BOB− anions were exchanged and incorporated into the PBTTT thin film.